How to Test Your Alternator's Voltage Regulator

Bad Voltage Regulator Symptoms

Symptoms of a bad voltage regulator may include:

• High voltage output
• Low voltage output
• No voltage output
• Lights dim or flicker
• Faulty high-beam headlamp bulbs
• Engine working erratically (weak or flickering ignition system)
• Adding water to the battery frequently
• Growing corrosion around battery terminals and top
• Dead battery
• Battery or check engine light indicator lit on dashboard

Keep in mind that some of these symptoms may come from loose or corroded charging system connections.

Therefore, Make Sure to Check For

• clean and tight battery connections,
• good battery cables,
• good engine and chassis ground connections and wires,
• a clean battery in good operating condition (have it tested if necessary), and
• the drive or serpentine belt is not worn and has proper tension.

These preliminary checks will help you eliminate the most common charging system problems.

1. What Does a Voltage Regulator Do?

Basically, the voltage regulator controls field current through the rotor, inside the alternator, in order to control alternator output.

Without a voltage regulator, an alternator may put out up to 250 volts. This is enough to destroy the car's battery and electrical system.

The voltage regulator is usually found inside or on the back of the alternator case. Increasingly, though, late-model vehicles have the engine control module (ECM) regulating alternator voltage output through a special circuit.

Older models used an electro-mechanical, external voltage regulator, mounted somewhere in the engine compartment.

On a computer-controlled charging system, the electronic or powertrain control module can monitor system operation, cut off charging output if voltage levels are too high, and trigger diagnostic trouble codes.

This is part of a fail-safe circuit in the computer and can greatly help you diagnose system problems and describe potential faults.

2. Voltage Regulator Test

Charging voltage can vary between 13.5 and 14.8 volts, depending on ambient temperature.

On a nice 70-degree F (21 C) day, you can expect a voltage regulator charging your car's battery at about 14.2 volts. And the higher the temperature, the lower the charging voltage.

This test is a simple procedure to check the alternator voltage regulator output. You need a digital multimeter for this test.

1. Set the parking brake and shift the transmission to Neutral (manual) or Park (automatic).
2. Set your multimeter to DC Voltage and select the 20 volts in the scale.
3. Connect the meter's red lead to the battery's positive (+) post and the meter's black lead to the battery's negative (-) post.
4. Notice the open-circuit voltage of the battery. Your battery should be at about 12.6 volts, 12.4 volts minimum; otherwise, charge the battery and continue with this test.
5. Now, ask an assistant to start the engine and run it at 1,500 rpm.
6. Take note of your voltmeter reading.

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A good output voltage should be about 2 volts higher than your battery's open-circuit voltage. Consult your vehicle repair manual, if necessary, to check the correct specifications for your particular model.

• If you noticed an output voltage reading below 13 volts right after starting the engine, there could be a charging system problem. Conduct a voltage drop test as described in the following subsection, Checking Wires Using Voltage Drop.
• If the output voltage reading is 16 volts or higher, there's an overcharging problem. This usually indicates a bad voltage regulator.
• If voltage seems to fluctuate during your test, switch your voltmeter to the AC voltage scale and take another output voltage reading with the engine still running.
  • This time, connect your meter's red lead to the B+ terminal on the back of the alternator, and the meter's black lead to battery negative (-).
  • Usually, the presence of 0.25 AC volts means a leaking diode that requires replacing the alternator. However, some manufacturers recommend replacing the alternator if 0.50 AC volts are detected.
  • However, if you have noticed engine performance issues, this might be the problem. Consult your vehicle repair manual for an acceptable diode leak rate if necessary.

If your output voltage is within specifications, continue with this test:

1. With the engine running, increase the engine speed to 2,000 rpm.
2. Turn on the headlights, AC, defogger, and other high-current accessories you may have.
3. Take note of your voltmeter reading.

The voltage output reading should be about 0.5 volts higher than your battery's open circuit voltage.

Most voltage regulators are calibrated to output between 13.5 and 15.5 charging volts on a fully charged battery at normal temperature with no accessories or lights on. Consult the specifications in your vehicle repair manual for your particular application.

Keep in mind that a worn or loose drive belt and other vehicle operating conditions, like high temperatures, can affect how the voltage regulator operates.

When your test shows a steady or intermittent high or low voltage output, the voltage regulator is possibly bad. Most voltage regulators fail by allowing a high voltage output, though. However, before going any further, check that all the connections to the alternator and battery are good and clean, as described in the next section.

a. Checking Wires Using Voltage Drop

A quick way to examine the wires and connections in the charging system is to check for voltage drops.

1. Set your voltmeter to 2 volts.
2. Start the engine and let it idle.
3. Measure for voltage across individual wires and connections in the charging system.
4. If there's voltage over 0.2 volts in any wire or connection, check for corroded, damaged, or loose wires.
5. When fixing wires and connections, aim for a voltage drop of less than 0.1 volts or 0.

This post on charging system voltage drop tests walks you through the steps in more detail. Pay special attention to the ground side of the charging system, which can cause trouble for the voltage regulator.

If the charging circuit connections are good, continue with the following tests. You can check whether your voltage regulator is at fault through a regulator bypass test, as described in the following section.

The following video gives you a charging system check overview that you can follow as well to test your charging system if necessary.

b. Voltage Regulator Bypass Test

On many alternators (except those with computer voltage regulation), you can bypass the voltage regulator to test whether your voltage regulator or some other component (alternator or charging circuit) is at fault.

There could be several ways to bypass the voltage regulator, depending on the charging system configuration for your particular vehicle model. Your voltage regulator may be connected to the power side of the circuit (type-B circuit) or the ground side (type-A circuit). Consult your repair manual.

• If the rear of your alternator has a 'test tab', you need to short this tab to the alternator frame using a screwdriver while checking voltage output at the battery with the engine running.
• On other systems, you may need to connect the battery and field terminals using a jumper wire while checking voltage output at the battery with the engine running.
• If your car's charging system is controlled through the computer, you may need a scan tool to check voltage regulation.

On modern Chrysler vehicles, most likely the computer regulates alternator output voltage. Depending on its particular configuration, you may test charging output by grounding the corresponding field terminal, or applying battery voltage to the appropriate terminal (on B-circuit types, 2002 and newer).

Follow These Precautions:

• During your test, keep all electrical accessories off before starting the engine.
• Disconnect the jumper wires once your digital multimeter reads charging voltage.
• Turn off the engine.

Consult the vehicle repair manual for your particular model to conduct this test, if necessary.

• When the voltage regulator is bypassed, you should see maximum voltage output.
• If voltage output remains at a normal level, most likely the voltage regulator is faulty.
• If voltage output remains at the same level (low, for example) as in your initial test (Section II, step 6), most likely you have a faulty alternator.

The next video shows you how to test an external voltage regulator and how to bypass it.

c. Voltage Regulator Adjustment

Some alternators with an old configuration allow adjusting of the voltage regulator. On these units, you can find a small adjusting screw on the voltage regulator.

1. Connect your voltmeter across the battery posts.
2. Set the Parking brake.
3. Shift the transmission to Neutral (manual) or to Park (automatic).
4. Start the engine and let it idle. Turn off any accessories, if necessary.
5. Check battery charging voltage.
6. Turn the adjusting screw using a small screwdriver to adjust charging voltage to specifications.

Refer to your vehicle repair manual to make sure you have an adjustable voltage regulator, locate the adjusting screw, and set voltage output to specifications.

d. Testing a Contact-Point Voltage Regulator

Old type DC generators and early vehicle charging systems used a contact-point type voltage regulator. Basically, it consisted of a coil, a set of points and resistors to control alternator voltage and current output. These regulators were replaced by electronic or solid-state voltage regulators.

Still, there might be some vehicles on the road today fitted with this type of regulator.

Usually, the contact points in the regulator are the ones to cause trouble after many miles of service due to wear or pitting.

To repair a contact-point voltage regulator:

• File, test and adjust the regulator points, as necessary.
• If still, voltage output is out of specifications, replace the regulator.

Refer to the repair manual for your particular vehicle make and model.

3. Voltage Regulator Replacement

To replace it, remove the voltage regulator from the back, or inside, of the alternator. Consult your vehicle repair manual for the procedure for your particular model.

If you don't have the manual yet, you can buy a relatively inexpensive copy online through Amazon. Haynes manuals come with step-by-step procedures for many maintenance, troubleshooting and component replacement projects you can do at home. So you'll recoup your small investment soon.

Some vehicle models use alternators with internal voltage regulation. You'll need to replace the alternator, if voltage regulation has failed.

The same goes for computer-controlled voltage regulation. When the voltage-regulator circuit fails, most likely you'll need to replace the Powertrain Control Module (PCM).

Probably your voltage regulator passed the tests. However, you still may suspect a problem in the system. In this case, this other post will help you check the charging system in your car.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2019 Dan Ferrell

Comments

Tomas Jacob Organ on August 30, 2020:

How many watts maximum can an alternator without a regulator generate? And how hard is it to get an alternator to its maximum rpm.

Dan Ferrell (author) on August 18, 2020:

When the regulator fails. Let's suppose it surpasses its maximum current (possibly 4 amps). At 12 volts, increasing rpm to its max you may get around 150, in some models over 200.

TJ Organ on August 17, 2020:

In what circomstance will an altinator qithout a regulator generate 250V

Dan Ferrell (author) on January 06, 2020:

Check the circuit between the alternator and the dashboard light. You may need the repair manual for your particular model to identify the wires and test. Hope this helps.